REDUCTION OF ENERGY DEMAND IN PAPER MAKING USING ONLINE OPTIMISATION AND CONTROL

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Paper making is an energy-intensive industry: even a small scale paper mill can use in excess of 150GWh of energy per year, as much as a medium-sized town. There are more than 50 paper mills in the UK with annual production of more then 5 million tonnes of paper and board. Paper mills produce products which range from fine papers to corrugated cardboard and involves fusion of a variety of processes with different characteristics, prominent amongst which are, non-minimum phase, batch, multivariable, time-delay and nonlinear system. The process consists of a number of sections arranged in a sequential way. The control of the whole production line is generally realized by distributed control systems (DCS). As such the plant has complex multilayered control systems with mixture of advanced, classical and manual loops in each layer. Moreover, paper-making machine is a hybrid system, combining wet end and dry end, which represents a mixture of batch and continuous processes. In this project, in parallel to the sectional process modelling, energy flow models will be established using principles in thermal dynamics (energy balance and entropy flow etc) and the process data available from the control systems along the paper making production line. It is expected that the energy flow models will reveal the energy distribution in the entire process and the sectional energy consumptions will be dynamically linked with control variables (i.e., control loop set points) and other sectional operational variables. As soon as the process and energy flow models of the plant are available, they can be used for the thermal energy management via online dynamic optimisation and control via model predictive control (MPC) techniques. In this context, MPC can be used to realize multi-optimization objectives in terms product quality, production and energy efficiency. At present, MPC has been used for real-time optimization (RTO) so as to generate appropriate set points to the control loops at the process level, and single performance function has been used together with process constraints. At present, multiple objectives can be incorporated into MPC algorithms via specifying their relative preferences prior to the optimization being performed. However, as the cost/energy consumption changes with time, this preference information will change dynamically and a range of feasible quality/efficiently trade-offs should be explicitly made available to the process supervisors.Based on the results of this research simple-to-use modelling and information presentation systems will be developed. The system will be developed in collaboration with the industrial partners involved in the project and will be applied to perform integrated process monitoring, control and optimisation of the exemplar processes to demonstrate the effectiveness of the developed methodologies. An online closed loop real-time multivariable energy-reducing MPC control system will also be implemented on two of the paper machines:1. The SK SSK board machine is typical of its kind: natural unregulated variations in sheet drainage occur and cause variations in the solids content of the sheet entering the dryer. The higher the solids content of the sheet entering the dryer, the more steam is required to dry the sheet. 2. On the other hand, at Aylesford, a contract with the energy supplier binds the mill to take a fixed quantity of steam each day. Improved thermal efficiency would result in steam having to be dumped unless production was increased to use the spare steam.